1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging (referred hereafter as MRI) system, and more particularly, to an intercommunication facility (referred hereafter as intercom) incorporated in the MRI system.
2. Description of the Background Art
The MRI system has been utilized for a medical diagnosis in order to obtain the tomographic image of the patient. In the medical diagnosis using the MRI system, the patient is placed inside the static magnetic field generated by the MRI apparatus, and the RF (radio frequency) pulses are applied from the RF coil of the MRI apparatus onto the patient placed in the static magnetic field so as to excite the hydrogen atoms within the patient. Then, the nuclear magnetic resonance signals (referred hereafter as MR signals) emitted from the patient as the excited hydrogen atoms return to their stable states are received from a desired tomographic plane specified by the gradient magnetic fields applied by the MRI apparatus onto the patient, and the tomographic image of the specified tomographic plane is reconstructed according the received MR signals.
In such an MRI system, the frame body of the MRI apparatus itself is located inside an isolated examination room and the controlling of the MRI apparatus is carried out from a system control unit located outside of the examination room.
Here, the examination room is usually equipped with the electromagnetic shielding because the presence of the external electromagnetic waves with the same frequency as the MR signals to be detected can cause the adverse effects such as the appearance of the artifact on the reconstructed tomographic images.
Consequently, the MRI system incorporates an intercom facility for enabling the communication between the interior of the examination room and the exterior of examination room.
Conventionally, the intercom facility provided in the MRI system has a configuration as shown in FIG. 1A, in which the intercom comprises a set of a microphone 1 and a loudspeaker 2 for transmitting sounds from the exterior of the examination room to the interior of the examination room which can be connected or disconnected by a switch 5, and a set of a microphone 3 and a loudspeaker 4 for transmitting sounds from the interior of the examination room to the exterior of the examination room which can be connected or disconnected by a switch 6, where the switches 5 and 6 are controlled by an intercom controller 7. In this FIG. 1A, the frame body 8 located inside the examination room and the system control unit 9 located outside the examination room are also depicted as separate entities independent from the above described intercom facility, although the intercom facility can be incorporated integrally within the frame body 8 and the system control unit 9 in practice.
Now, in this conventional intercom facility shown in FIG. 1A, the intercom controller 7 controls the ON/OFF of the switches 5 and 6 according to the timing chart shown in FIG. 1B, in which the ON/OFF states 11 and 12 of the switches 6 and 5, respectively, are indicated along with the reading gradient field pulse 10 of the pulse sequence to be executed. Namely, the switch 6 for a sound transmission from the interior to the exterior is normally put in the ON state, as the intercom facility cannot be controlled from the interior of the examination room, such that the sound inside the examination room is audible at the exterior of the examination room. While the switch 6 is in the ON state, the switch 5 for a sound transmission from the exterior to the interior remains in put the OFF state so that the sound outside of the examination room is inaudible at the interior of the examination room. When the switch 5 is turned into the ON state as the communication from the exterior of the examination room to the interior of the examination room is required, the switch 6 is turned into the OFF state such that the speech uttered at the exterior of the examination room becomes audible at the interior of the examination room. During the application of the reading gradient field pulse 10, the switch 6 remains in the ON state while the switch 5 remains in the OFF state normally, as indicated in FIG. 1B.
However, in the MRI apparatus, the application of the reading gradient field pulses causes the vibrations on the bobbin around which the gradient field coils are wound, and these vibrations in turn causes a large noise, so that the large noise due to the application of the reading gradient field pulses is also transmitted from the interior to the exterior through the intercom facility as the switch 5 remains in the ON state during the application of the reading gradient field pulses. This problem becomes prominent in the high speed type MRI apparatus that is under the development in recent years, because the imaging pulse sequence for the high speed imaging scheme such as the echo planar imaging uses the strong reading gradient field which is to be repeatedly reversed in rapid successions.
As a consequence, the hearing of the operator at exterior of the examination room can be considerably disturbed by the large noise transmitted through the intercom facility to a level at which the correct hearing of the speech uttered by the patient inside the examination room becomes difficult. Moreover, in some cases, the large noise transmitted through the intercom facility can be even harmful to the operator's ears.